Gap control via overmold teeth and hard stops

ABSTRACT

A forceps includes an end effector assembly having a stop and a plurality of overmold teeth within at least one jaw member. One (or both) of the jaw members is moveable relative to the other between a spaced-apart position and an approximated position for grasping tissue therebetween. One (or both) of the jaw members includes a stop molded within an insulative housing, and an insulator plate with the overmold teeth formed from plastic. The overmold teeth extend through openings within a sealing plate and protrude past the tissue sealing surface of the sealing plate. The stop primarily controls the gap distance between opposing jaw members by bearing most of an applied load and the overmold teeth assist in controlling the gap distance by bearing the remaining applied load.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patent application Ser. No. 14/578,953 (now U.S. Pat. No. 9,192,434), filed on Dec. 22, 2014, which is a continuation application of U.S. patent application Ser. No. 13/835,004, filed on Mar. 15, 2013, now U.S. Pat. No. 8,939,975, which claims the benefit of and priority to U.S. Provisional Application Ser. No. 61/672,347, filed on Jul. 17, 2012, the entire contents of each of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to surgical instruments and, more particularly, to a surgical instrument for controlling gap distance between jaw members using hard stops and overmold teeth.

2. Background of Related Art

Electrosurgical instruments, e.g., electrosurgical forceps, utilize both mechanical clamping action and electrical energy to effect hemostasis by heating tissue to coagulate and/or cauterize tissue. Certain surgical procedures require more than simply cauterizing tissue and rely on the unique combination of clamping pressure, precise electrosurgical energy control and gap distance (i.e., distance between opposing jaw members when closed about tissue) to “seal” tissue.

One method of controlling the gap distance, uses one or more ceramic dots on one or both jaw members. The ceramic dots are deposited atop one or both jaw members. The ceramic dots may be vapor deposited onto sealing plates. The ceramic dots project from the tissue engaging surface of one or both jaw members and the ceramic dots form a corresponding series of nonconductive stop members for controlling the separation distance between opposing jaw members when closed about tissue. Most ceramics are stable at elevated temperatures and usually exhibit low thermal and electrical conductivities. In addition, ceramic materials have high melting points and are resistant to oxidation, corrosion, or other forms of degradation to which metals are usually more prone. However, ceramic dots add substantial cost to the manufacture of the jaw members.

SUMMARY

As used herein, the term “distal” refers to the portion that is being described which is further from a user, while the term “proximal” refers to the portion that is being described which is closer to a user.

In accordance with one aspect of the present disclosure, a forceps includes an end effector assembly having a stop and a plurality of overmold teeth within at least one jaw member. One (or both) of the jaw members may be moveable relative to the other between a spaced-apart position and an approximated position for grasping tissue therebetween. One (or both) of the jaw members includes a stop molded within an insulative housing, and an insulator plate with the overmold teeth formed from plastic. The overmold teeth extend through openings within a sealing plate and protrude past the tissue sealing surface of the sealing plate. The stop primarily controls the gap distance between opposing jaw members by bearing most of an applied load and the overmold teeth assist in controlling the gap distance by bearing the remaining applied load.

According to an aspect of the present disclosure, an end effector assembly includes a pair of opposing jaw members configured to primarily control a gap distance between opposing jaw members. At least one of the jaw members includes an insulative base including a hard stop. The hard stop is configured to primarily control a gap distance between the opposing jaw members. At least one of the jaw members also includes a support base coupled to the insulative housing and an insulative plate coupled to the support base and formed with a plurality of overmold teeth and a sealing plate mounted to the insulative plate. The sealing plate includes a plurality of openings formed therein. The plurality of overmold teeth extend through the corresponding plurality of openings on the sealing plate and are configured to assist in controlling the gap distance between opposing jaw members.

According to aspects of the present disclosure, the hard stop may be remotely disposed relative to the sealing plate.

According to other aspects of the present disclosure, the plurality of overmold teeth may be configured to contact the corresponding plurality of overmold teeth on the opposing jaw member. Alternatively, the plurality of overmold teeth may be configured to contact the sealing plate on the opposing jaw member. The plurality of overmold teeth may also be located along a blade slot defined in the seal plate to facilitate grasping of tissue during tissue division.

According to a further aspect of the present disclosure, the hard stop may be configured to primarily control the gap distance by bearing most of the applied load as the end effector assembly grasps tissue.

According to another aspect of the present disclosure, the hard stop may be engaged when jaw members flex under the applied load.

According to yet another aspect of the present disclosure, a method of forming a jaw member of an end effector includes the steps of forming a support base and forming an insulative plate with a plurality of overmold teeth. The method further includes the steps of forming a sealing plate with a plurality of openings and mounting the insulative base to the support base. The method further includes the step of mounting the sealing plate onto the insulative plate with the plurality of overmold teeth extending through the plurality of openings on the sealing plate. The method further includes the step of overmolding an insulative housing with a hard stop around the support base to form the jaw member. When the end effector is closed around tissue the hard stop is configured to bear the majority of an applied load and the overmold teeth bear a smaller remaining portion of the applied load

The method may further include that the insulative plate may be formed by injection molding. The method may also include that the hard stop may be remotely disposed relative to the sealing plate.

According to another aspect of the present disclosure, an end effector assembly includes a pair of opposing jaw members. At least one of the jaw members includes an insulative housing including a hard stop formed from a plastic material. The hard stop may be configured to bear the majority of an applied load as the end effector assembly is closed around tissue. At least one of the jaw members further includes a support base coupled to the insulative housing and an insulative plate molded from the plastic material with a plurality of overmold teeth and a sealing plate mounted to the insulative plate. The sealing plate includes a plurality of openings formed therein. The plurality of overmold teeth extend through the corresponding plurality of openings on the sealing plate past a tissue sealing surface of the sealing plate. The plurality of overmold teeth may be configured to ensure that the opposing jaw members are an appropriate gap distance apart.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure are described herein with reference to the drawings wherein:

FIG. 1 is a front, perspective view of an endoscopic surgical instrument configured for use in accordance with the present disclosure;

FIG. 2 is a front, perspective view of an open surgical instrument configured for use in accordance with the present disclosure;

FIG. 3 is a front, perspective view of one embodiment of a jaw member configured for use with the surgical instrument of FIG. 1 or 2;

FIG. 4 is a side, perspective view of an end effector assembly configured for use with the surgical instrument of FIG. 1 or 2;

FIG. 5 is cross-sectional view of an end effector assembly configured for use with the surgical instrument of FIG. 1 or 2;

FIGS. 6A and 6B are exploded views of the opposing jaw members of FIG. 4;

FIGS. 7A-7C are front, perspective views of different embodiments of an end effector assembly configured for use with the surgical instrument of FIG. 1 or 2; and

FIG. 8 is a flowchart of a method for forming a jaw member according to the present disclosure.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described in detail with reference to the drawing figures wherein like reference numerals identify similar or identical elements.

Referring now to FIGS. 1 and 2, FIG. 1 depicts a forceps 10 for use in connection with endoscopic surgical procedures and FIG. 2 depicts an open forceps 10′ contemplated for use in connection with traditional open surgical procedures. For the purposes herein, either an endoscopic instrument, e.g., forceps 10, or an open instrument, e.g., forceps 10′, may be utilized in accordance with the present disclosure. Obviously, different electrical and mechanical connections and considerations apply to each particular type of instrument; however, the novel aspects with respect to the end effector assembly and the operating characteristics thereof remain generally consistent with respect to both the open and endoscopic configurations.

Turning now to FIG. 1, an endoscopic forceps 10 is provided defining a longitudinal axis “X-X” and including a housing 20, a handle assembly 30, a rotating assembly 70, a trigger assembly 80, an actuator 90, and an end effector assembly 100. Forceps 10 further includes a shaft 12 having a distal end 14 configured to mechanically engage end effector assembly 100 and a proximal end 16 that mechanically engages housing 20. Housing 20 contains the internal working components of the forceps 10 which are not described herein but which may be found in commonly-owned U.S. Pat. No. 7,156,846, the entire contents of which are hereby incorporated by reference herein.

End effector assembly 100 is shown attached at the distal end 14 of shaft 12 and includes a pair of opposing jaw members 110 and 120. Jaw members 110, 120 are moveable between a spaced-apart position and an approximated position for grasping tissue therebetween. End effector assembly 100 is designed as a unilateral assembly, e.g., where jaw member 120 is fixed relative to shaft 12 and jaw member 110 is moveable about pivot 103 relative to shaft 12 and fixed jaw member 120. However, end effector assembly 100 may alternatively be configured as a bilateral assembly, e.g., where both jaw member 110 and jaw member 120 are moveable about a pivot 103 relative to one another and to shaft 12.

With continued reference to FIG. 1, forceps 10 also includes electrosurgical cable 610 that connects forceps 10 to a generator (not shown) or other suitable power source, although forceps 10 may alternatively be configured as a battery powered instrument. Cable 610 includes a wire (or wires) (not explicitly shown) extending therethrough that has sufficient length to extend through shaft 12 in order to provide electrical energy to at least one of the jaw members 110 and 120 of end effector assembly 100. Trigger 82 of trigger assembly 80 may be selectively depressed to advance a knife (not shown) between jaw members 110, 120 to cut tissue grasped therebetween. Actuator 90, on the other hand, is selectively activatable to supply electrosurgical energy to one (or both) of jaw members 110, 120, as will be described in greater detail below.

With continued reference to FIG. 1, handle assembly 30 includes fixed handle 50 and a moveable handle 40. Fixed handle 50 is integrally associated with housing 20 and handle 40 is moveable relative to fixed handle 50. Rotating assembly 70 is rotatable in either direction about a longitudinal axis “X-X” to rotate end effector 100 about longitudinal axis “X-X.” Moveable handle 40 of handle assembly 30 is ultimately connected to a drive assembly (not shown) that, together, mechanically cooperate to impart movement of jaw members 110 and 120 between the spaced-apart position and the approximated position to grasp tissue disposed between jaw members 110, 120. As shown in FIG. 1, moveable handle 40 is initially spaced-apart from fixed handle 50 and, correspondingly, jaw members 110, 120 are in the spaced-apart position. Moveable handle 40 is depressible from this initial position to a depressed position corresponding to the approximated position of jaw members 110, 120.

Referring now to FIG. 2, an open forceps 10′ is shown including two elongated shafts 12 a and 12 b, each having a proximal end 16 a and 16 b, and a distal end 14 a and 14 b, respectively. Similar to forceps 10 (FIG. 1), forceps 10′ is configured for use with end effector assembly 100. More specifically, end effector assembly 100 is attached to distal ends 14 a and 14 b of shafts 12 a and 12 b, respectively. As mentioned above, end effector assembly 100 includes a pair of opposing jaw members 110 and 120 that are pivotably connected about a pivot 103. Each shaft 12 a and 12 b includes a handle 17 a and 17 b disposed at the proximal end 16 a and 16 b thereof. Each handle 17 a and 17 b defines a finger hole 18 a and 18 b therethrough for receiving a finger of the user. As can be appreciated, finger holes 18 a and 18 b facilitate movement of the shafts 12 a and 12 b relative to one another that, in turn, pivots jaw members 110 and 120 from an open position, wherein the jaw members 110 and 120 are disposed in spaced-apart relation relative to one another, to a closed position, wherein the jaw members 110 and 120 cooperate to grasp tissue therebetween.

A ratchet 30′ may be included for selectively locking the jaw members 110 and 120 relative to one another at various positions during pivoting. Ratchet 30′ may include graduations or other visual markings that enable the user to easily and quickly ascertain and control the amount of closure force desired between the jaw members 110 and 120.

With continued reference to FIG. 2, one of the shafts, e.g., shaft 12 b, includes a proximal shaft connector 19 that is configured to connect the forceps 10′ to a source of electrosurgical energy such as an electrosurgical generator (not shown). Proximal shaft connector 19 secures an electrosurgical cable 610′ to forceps 10′ such that the user may selectively apply electrosurgical energy to jaw member 110 and/or jaw member 120 of end effector assembly 100.

Referring now to FIGS. 3-5, one embodiment of jaw members 210 and 220 is provided in accordance with the present disclosure. FIG. 3 shows a front perspective view of jaw member 220. Jaw member 220 includes an insulative housing 227, support base 230, insulative plate 222, and sealing plate 240. Molded within the insulative housing 227 is a hard stop 225 configured to limit the gap distance when jaw members 220 and 210 (see FIG. 4) are closed around tissue. Additionally, overmold teeth 235 a-235 e assist in limiting the gap distance “G” (See FIG. 4) when jaw members 210 and 220 are closed around tissue.

One or more overmold teeth 235 a-235 e on jaw member 220 contact one or more respective opposing overmold teeth 285 a-285 e (see FIG. 4) on jaw member 210 as the jaw members 210, 220 are closed due to a tip-bias. Then as the jaw members 210, 220 flex one or more hard stops 225, and/or 275 are engaged. When the hard stops 225 and/or 275 are engaged, hard stops 225 and/or 275 bear most of the load. By having the one or more overmold teeth 235 a-235 e, 285 a-285 e contact first ensures that the jaw members are at the appropriate gap distance “G” (see FIG. 4). In other words, the tip-bias ensures that the jaw members 210, 220 are properly closing.

Alternatively, hard stop 225 and/or 275 may be configured to control the initial gap distance between jaw members 210 and 220 and to bear most of the load as the tissue is compressed between jaw members 210 and 220 while overmold teeth 235 a-235 e control the gap distance while jaw members 210 and 220 flex as they seal tissue.

As the jaw members 210, 220 clamp together around tissue, hard stop 225 and/or hard stop 275 (See FIG. 6A) and overmold teeth 235 a-235 e and/or 285 a-285 e maintain the gap distance “G” with the hard stop 225 and/or 275 bearing most of the applied load. The gap distance is about 0.001 inches to about 0.005 inches.

Hard stop 225 may be disposed at a remote location or away from the high temperatures of seal plate 240 (e.g., closer to proximal end 221 of jaw member 220) to reduce deflection of hard stop 225 under loading. By hard stops 225 and/or 275 being removed from the high temperatures of the seal plates 240, 312, the hard stops 225 and/or 275 can bear a majority of the applied load when a user grasps tissue with end effector 200 without the unnecessary risk of melting or deflection.

The overmold teeth 235 a-235 e and/or 285 a-285 e may be used to assist the user in gripping tissue during grasping. The overmold teeth 235 a-235 e and/or 285 a-285 e are relatively small in size to reduce the effect of the overmold teeth 235 a-235 e and/or 285 a-285 e on tissue sealing performance. For example the overmold teeth may range from about 0.020 inches to about 0.050 inches in diameter. However, the size of the teeth can vary based on the size of the jaw members. Initially, one or more overmold teeth 235 a-235 e and/or 285 a-285 e may be used to check that jaw members 210, 220 are closing to the gap distance “G”. Then, as the jaw members 210, 220 flex then hard stops 225 and/or 275 make contact and bear most of the load. Alternatively, the overmold teeth 235 a-235 e and/or 285 a-285 e may be used to secondarily control the gap distance “G” as the jaw members 220, 210 flex. For example, when the jaw members flex 220, 210 under a particular loading condition, only one overmold tooth 235 a may make contact with a corresponding opposing overmold tooth 285 a. Alternatively, when the jaw members 220, 210 are under a different loading condition, more overmold teeth 235 a-235 e on jaw member 220 may make contact with corresponding overmold teeth 285 a-285 e on jaw member 210, however not all overmold teeth 235 a-235 e and/or 285 a-285 e need to contact each other to maintain proper gap distance “G”.

Turning to FIGS. 6A and 6B, the opposing jaw members 210 and 220 include support bases 319 and 230 that extend distally from flanges 313 and 221, respectively. The support bases 319 and 230 are configured to support insulative plates 322 and 222, which, in turn, support electrically conductive sealing plates 312 and 240 thereon. Sealing plates 312 and 240 may be affixed atop the insulative plates 322 and 222, respectively, and support bases 319 and 230, respectively, in any suitable manner including snap-fit, over-molding, stamping, ultrasonically welded, etc. The support bases 319 and 230, insulative plates 322 and 222, and sealing plates 312 and 240 are encapsulated by the outer insulative housings 316 and 227 by way of a subsequent overmolding process. The jaw members 210 and 220 are connected via an ultrasonic weld or other suitable joining process to electrical jaw leads 325 a and 325 b, respectively.

The jaw members 210 and 220 also include proximal flanges 313 and 221 extending proximally from the support bases 319 and 230, respectively, each of which includes an elongated angled cam slot 317 and 327, respectively, defined therethrough. The electrically conductive sealing plates 312 and 240 and the insulator plates 322 and 222 include respective longitudinally-oriented knife slots 315 a, 315 a′ and 315 b, 315 b′, respectively, defined therethrough for reciprocation of the knife blade (not shown). Jaw member 220 further includes one or more overmold teeth 235 a-235 e disposed on the inner facing surface of insulative plate 222 to define a gap between opposing jaw members 210 and 220 during sealing and/or cutting of tissue. The overmold teeth 235 a-235 e are molded within insulative plate 222 when the insulative plate 222 is molded. Types of plastic material that may be used are Amodel®, Trogamid®, PEKK, G-PEAK, PEEK, Thermotuff™, Ultem®, etc., all of which may be mineral and/or fiber reinforced.

The overmold teeth 235 a-235 e may be located along blade slot 315 b′. The overmold teeth 235 a-235 e extend through openings 237 a-237 e within seal plate 240 and are slightly higher in elevation than seal plate 240 to prevent seal plates 312 and 240 from touching and creating a short between the seal plates 312, 240. Additionally, when the overmold teeth 235 a-235 e are located along blade slot 315 b′, the overmold teeth 235 a-235 e help grip tissue closer to where the division takes place and may produce a more reliable cut even when a blade (not shown) is not as sharp. Additionally, if insulator plate 322 includes one or more overmold teeth 285 a-285 e, then overmold teeth 285 a-285 e extend through openings 287 a-287 e within seal plate 312 and are slightly higher in elevation than seal plate 312 to prevent seal plates 312 and 240 from touching. Overmold teeth 285 a-285 e are formed in the same manner used to create overmold teeth 235 a-235 e. Additionally, if overmold teeth 285 a-285 e are spaced apart along blade slot 315 a′, then overmold teeth 285 a-285 e assist in gripping tissue closer to where the division takes place.

Referring to FIGS. 7A-7C, the overmold teeth 235 a-235 e and/or 285 a-285 e may be located in any location along insulative plates 222, 322 and either insulative plate 222 or 322 may include one or more overmold teeth 235 a-235 e and/or 285 a-285 e. FIG. 7A shows one embodiment of an end effector assembly 700 where the overmold teeth 235 a-235 e on jaw member 220 contact the mating row of overmold teeth 285 a-285 e on jaw member 210. With end effector 700, the overmold teeth 235 a-235 e and/or 285 a-285 e are almost always contacting plastic. FIG. 7B shows an alternative embodiment with end effector assembly 710, with overmold teeth 265 a-265 g on alternating sides blade slot 315 b (See FIG. 6B). With end effector 710, each tooth of the overmold teeth 265 a-265 g may contact directly against seal plate 312 because of the symmetry with overmold teeth (not shown) on jaw member 210. However, if overmold teeth 265 a-265 g are of opposite symmetry to overmold teeth (not shown) on jaw member 210, then each tooth may contact an opposite overmold tooth, i.e. contact plastic and not a sealing plate 240, 312. Additionally, if only one jaw member 210 or 220 has overmold teeth 235 a-235 e or 285 a-285 e, then one or more teeth of the overmold teeth 235 a-235 e or 285 a-285 e contact the opposite seal plate 240, 312.

FIG. 7C shows an alternative end effector assembly 720. End effector assembly 720 includes a row of overmold teeth 295 a-295 g along a first side 725 of blade slot 315 b on jaw member 220 and a row of overmold teeth 297 a-297 g along a second side 727 of blade slot 315 a on jaw member 210. When end effector assembly 720 is closed around tissue, overmold teeth 295 a-295 g and 297 a-297 g contact directly against seal plate 312, 240 on respective opposing jaw members 210, 220. Alternatively, end effector 720 may be configured with a row of overmold teeth 295 a-295 g along a first side 725 of blade slot 315 b on jaw member 220 and a row of overmold teeth 297 a-297 g along a first side 726 of blade slot 315 a on jaw member 210. In this alternative embodiment, the overmold teeth 295 a-295 g and 297 a-297 g directly oppose each other and are almost always contacting plastic.

FIG. 8 is a flow diagram of process 800 for forming a jaw member 210, 220. The process 800 starts at step 805, and at step 810 the support base 230, 319 is formed. The support base 230, 319 may be formed of a plastic material by an injection molding process. Next, at step 820, an insulative plate 222, 322 with a plurality of overmold teeth 235 a-235 e, 285 a-285 e is formed of a plastic material by an injection molding process. Then at step 830, the sealing plate 240, 312 is formed from a conductive material with a plurality of openings 237 a-237 e, 287 a-287 e. Next at step 840, the insulative plate 222, 312 is mounted to the support base 230, 319. Then at step 850, the sealing plate 240, 312 is mounted to the insulative plate 222, 312 with the plurality overmold teeth 235 a-235 e, 285 a-285 e extending through the plurality of openings 237 a-237 e, 287 a-287 e, respectively. The sealing plate 240, 312 may be affixed atop the insulative plate 222, 312 in any known manner in the art, snap-fit, overmolding, stamping, ultrasonically welded, etc. The process 800 ends at step 865 after the insulative housing 227, 316 is formed around support base 230, 319 at step 860. When the insulative housing 227, 316 is formed hard stop 225, 275 is also formed of a plastic material. One method for forming the insulative housing 227, 316 is by an overmolding process.

From the foregoing and with reference to the various figure drawings, those skilled in the art will appreciate that certain modifications can also be made to the present disclosure without departing from the scope of the same. While several embodiments of the disclosure have been shown in the drawings, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto. 

What is claimed is:
 1. An end effector assembly, comprising: a pair of opposing jaw members, at least one of the pair of opposing jaw members including: an insulative material; a plurality of teeth projecting from the insulative material and configured to control a gap distance between the pair of opposing jaw members; and a sealing surface defining: a longitudinally extending blade slot; and a plurality of openings located along the blade slot, each of the plurality of teeth disposed within a respective one of the plurality of openings and in communication with the blade slot.
 2. The end effector assembly according to claim 1, wherein each of the plurality of openings has an arcuate outer portion that terminates at the blade slot.
 3. The end effector assembly according to claim 1, wherein the plurality of teeth includes: a first row of teeth disposed on a first side of the blade slot; and a second row of teeth disposed on a second side of the blade slot.
 4. The end effector assembly according to claim 3, wherein the first row of teeth and the second row of teeth are disposed symmetrically about the blade slot.
 5. A jaw member for an end effector assembly, the jaw member comprising: an insulative material; a plurality of teeth projecting from the insulative material; and a sealing surface including: a longitudinally extending blade slot defined therein, the plurality of teeth at least partially defining the blade slot; and a plurality of openings located along the blade slot, each of the plurality of teeth disposed within and extending through a respective one of the plurality of openings.
 6. The jaw member according to claim 5, wherein each of the plurality of teeth has a planar inner portion that at least partially defines the blade slot.
 7. The jaw member according to claim 5, wherein the plurality of teeth includes: a first row of teeth disposed on a first side of the blade slot; and a second row of teeth disposed on a second side of the blade slot.
 8. The jaw member according to claim 7, wherein the first row of teeth and the second row of teeth are disposed symmetrically about the blade slot.
 9. An end effector assembly, comprising: a pair of opposing jaw members, at least one of the pair of opposing jaw members including: a sealing surface defining: a blade slot; and a plurality of openings located along and in communication with the blade slot; an insulative material; and a plurality of teeth projecting from the insulative material and protruding past the sealing surface to control a gap distance between the pair of opposing jaw members, each of the plurality of teeth disposed within a respective one of the plurality of openings.
 10. The end effector assembly according to claim 9, wherein the plurality of openings includes a peripheral edge having an arcuate configuration, a first end and a second end of the peripheral edge terminating at the blade slot.
 11. The end effector assembly according to claim 9, wherein the plurality of teeth includes: a first row of teeth disposed on a first side of the blade slot; and a second row of teeth disposed on a second side of the blade slot.
 12. The end effector assembly according to claim 11, wherein the first row of teeth and the second row of teeth are disposed symmetrically about the blade slot. 